The invention concerns a method for quantification of the level of weather and sea noise during marine seismic surveys In particular it concerns a method for measurement, treatment and quantification of weather and sea noise in order to evaluate the effect of noise on the results of the seismic survey.
For definitions of terms and explanations of well known systems and methods used in seismic surveys, reference is made to "Encyclopedic Dictionary of Exploration Geophysics", 2. ed., complied by R. E. Sheriff, and published by the Society of Exploration Geophysicist in 1984, and "Introduction to Geophysical Prospecting", M. B. Dobrin and Carl H. Savit, 4. ed., McGraw-Hill, Singapore 1988, particularly Chapters 5 and 7. The latter publication may be regarded as a standard text in the field of geophysical prospecting, including seismic exploration.
It is a well-known fact that weather conditions affect the quality of the reflected seismic signals which are recorded by the hydrophone groups during a marine seismic survey. During bad weather conditions, waves and current generate noise which can be recorded by the hydrophones and in addition noise is also generated in the water by the streamer cable, e.g. turbulence noise and other flow noise. It is normal procedure, therefore, to evaluate the conditions in the survey area and possibly interrupt the survey when the noise conditions are considered to be unfavorable. Investigations have also shown that the noise picture is altered after the data have been processed at the processing center. Noise specifications for seismic data acquisition should therefore be related to the effect of the noise on the data after processing. In this connection, special consideration should be given to the ability of stacking to remove noise and thus improve the signal/noise ratio. Consideration should also be given to the fact that the signal level in the survey area can vary greatly from survey to survey.
It is a well-known fact that the strength of the seismic reflection signal varies greatly with parameters such as the geological conditions in the survey area, the distance to the target and the strength of the seismic energy source used.
The strength of the seismic signal can be determined by a number of methods, e.g. by analyzing data from previous surveys in the same area, by analysing brute stacks for common reflection points aboard the survey vessel, or somewhat more directly from a slightly processed shot record.
As a rule, such methods will give a good picture of the signal level in the final section, in that by processing, attempts are made to maintain the signal level, but to reduce the noise, i.e. improve the signal/noise ratio.
However, it will be desirable to be able to quantify, i.e., determine the quantity of, the noise level in processed or stacked data from measurements made during the actual data acquisition. A general problem in this connection is that a data acquisition has to be carried out from a full line of noise records only in order to generate a CMP stack and thus obtain a good picture of the noise level after stacking.
These noise records could be taken during the period between the recording of a single shot and the next firing. A typical operation is conducted with a 10 second shot interval and a subsequent recording period of 6-7 seconds duration However, it is difficult to make sure whether the noise recorded actually is wind or weather noise and does not partly contain residual energy from the previous shot. In all probability this will depend on the geology of the area and possibly also other geographical parameters.
It should be mentioned here that a CMP stack is a summation of traces which represent the same reflection point in the subsurface. Such a CMP stack is generated by the summation of the traces from various shots with different travelling times between source and hydrophone group. The number of folds, i.e. the number of traces which cover the same common reflection point or CMP, will depend on the number of hydrophone groups and the distance between each shotpoint. Even though there is equipment aboard the survey vessel to perform data processing such as a simple CMP stacking, in reality it will be difficult to collect a sufficient amount of noise records to correspond to a complete CMP gather.